The humble sign above the door of the entrance can be seen on the right.
The Mithraeum is a subterranean temple devoted to the pagan god Mithras.

Mosas Hadas wrote quite eloquently about Mithraism.
Here is a quote from his book, Imperial Rome: "It
was one of three eastern religions which had an important influence in
Rome. It was the most popular of the three and had, by far, the highest
moral tone. The origins of the God Mithras are shrouded in antiquity;
by the time the Romans met him he had become associated with Persian Zoroastrianism,
and was a deity of truth and light. Mithraism introduced Romans to
the idea of a world divided between good and evil, the powers of light
and the powers of darkness. Its followers joined Mithras in his militant
fight against evil.

Mithraism was a man's religion
(perhaps
it's fatal flaw -dp) rigorous
and virile, with orders of membership somewhat similar to the degrees of
modern Freemasonry. With its sense of fraternity and its emphasis
on combat in the cause of good, the cult of Mithras spread like wildfire
through the Roman army; by the Third Century AD it was practically the
army's unofficial religion. Even the Emperor Commodus was reputed
to have been an adherent. In certain superficial elements its rites
were not unlike Christianity's; there was, for example, a ceremony similar
to baptism. In fact the two were, for a time, competitors for the
religious affections of Rome."

On the left you can see a patched wall. This is where the Mithraeum was discovered. Men digging a well in search of water instead found
this ancient chamber. (2.5m x 5m)

The black and white picture in the lower left may be a depiction of ceremonies
performed in the temple.

The picture below shows Mithras slaying the bull. As the story goes...
Mithras and the bull were alone on a barren world. Mithras killed
the bull to release the life which was within it. As the bull's blood
spilled upon the earth all of the forms of life we know today sprung forth
into existence. (Nucci)

Note:
If you find yourself in Capua, and you would like to visit the Mithraeum
you must ask the ticket seller at the amphitheater to take you there.